Magnetic random access memory (MRAM) that incorporates a MTJ as a memory storage device is a strong candidate to provide a high density, fast (1-30 ns read/write speed), and non-volatile solution for future memory applications. An MRAM array is generally comprised of an array of parallel first conductive lines on a horizontal plane, an array of parallel second conductive lines on a second horizontal plane spaced above and formed in a direction perpendicular to the first conductive lines, and a MTJ interposed between a first conductive line and a second conductive line at each crossover point. A first conductive line may be a word line while a second conductive line is a bit line or vice versa. Alternatively, the first conductive line may be a sectioned line which is a bottom electrode. There are typically other devices including transistors and diodes below the array of first conductive lines.
The MTJ consists of a stack of layers with a configuration in which two ferromagnetic layers are separated by a thin insulating layer such as Al2O3, AlNxOy, or NiOx which is called a tunnel barrier layer. One of the ferromagnetic layers is a pinned layer in which the magnetization (magnetic moment) direction is more or less uniform along a preset direction and is fixed by exchange coupling with an adjacent anti-ferromagnetic (AFM) pinning layer. The second ferromagnetic layer is a free layer in which the magnetization direction can be changed by external magnetic fields. The magnetization direction of the free layer may change in response to external magnetic fields which can be generated by passing currents through the conductive lines as in a write operation. When the magnetization direction of the free layer is parallel to that of the pinned layer, there is a lower resistance for tunneling current across the insulating layer (tunnel barrier) than when the magnetization directions of the free and pinned layers are anti-parallel. The MTJ stores digital information (“0” and “1”) as a result of having one of two different magnetic states.
In a read operation, the information is read by sensing the magnetic state (resistance level) of the MTJ through a sensing current flowing through the MTJ, typically in a current perpendicular to plane (CPP) configuration. During a write operation, the information is written to the MTJ by changing the magnetic state to an appropriate one by generating external magnetic fields as a result of applying bit line and word line currents. Cells which are selectively written to are subject to magnetic fields from both a bit line and word line while adjacent cells (half-selected cells) are only exposed to a bit line or a word line field. Due to variations in MTJ size and shape that affect the switching field of a free layer, a magnetic state in a half-selected cell may be undesirably altered when writing to a selected cell.
To preserve data (magnetic state) against erasure, an in-plane magnetic anisotropy has to be strong enough in the storing magnetic layer. Current designs are based on shape anisotropy involving rectangular, ellipse, eye, and diamond-like patterns. Coercivity in these designs is highly dependent on shape, aspect ratio, and MTJ cell size and is therefore very sensitive to cell shape and edge shape which are subject to variations because of cell patterning processes. As a result, MTJ cell differences can make the switching field highly variable and difficult to control.
Referring to FIG. 1, a conventional MTJ 1 is shown between a first conductive layer 2 and a second conductive layer 9. The MTJ 1 is comprised of a seed layer 3, an anti-ferromagnetic (AFM) layer 4, a ferromagnetic (pinned) layer 5, a tunnel barrier layer 6, a ferromagnetic (free) layer 7, and a capping layer 8. A sensing current 10 is shown in a CPP configuration along the z-axis. In the quiescent state, the free layer magnetization lies along the orientation of the pinned layer, either parallel or anti-parallel to the pinned layer magnetization. In other words, in an example where the magnetic direction of the pinned layer 5 is aligned along the +x direction, the magnetic direction of the free layer 7 may be oriented along either the +x or −x direction. Storage of the digital information is thus provided by the direction of the free layer magnetization.
Referring to FIG. 2, the resistance of a MTJ element is shown as a function of the external field along the orientation of the pinned layer magnetization. When the field is off, the two states with minimum and maximum resistance correspond to the free layer magnetization being parallel and anti-parallel, respectively, to the pinned layer magnetization. The field (Hs) required to switch between the two states is determined by the anisotropy energy which is related to shape anisotropy, for example, of the element.
The MTJ configuration depicted in FIG. 1 has several shortcomings with regard to MRAM applications. The coupling between the free layer and the pinned layer due to roughness of the tunnel barrier (oxide) layer is often called the orange peel effect. This coupling induces a bias in the switching threshold of free layer magnetization. Variations in this coupling cause undesirable variations in the switching threshold. A second problem with conventional MTJs is that magnetic charges at the edges of the pinned layer produce a bias and variations in this bias also lead to variations in the switching threshold. Another issue is that in order to achieve reliable switching behavior, the free layer is generally limited to materials with small coercivity (Hc). However, materials with small Hc typically do not produce a sufficiently high magnetoresistive (MR) ratio to meet high performance requirements. On the other hand, materials such as CoFeB and CoFe with high Fe content that are desirable for high MR ratios do not have the necessary magnetic softness for low coercivity. Therefore, a novel MTJ configuration is needed to overcome these shortcomings in state of the art MRAM devices.
U.S. Pat. No. 6,844,202 discloses a sensor to detect the presence of magnetic particles that are essentially paramagnetic such that their magnetization is a function of the external magnetic field. The sensor element is a planar layer with a circular magnetic moment that changes to a radial direction due to a radial fringing field of the magnetic particles.
A MTJ is disclosed in U.S. Pat. No. 6,730,395 and in related U.S. Patent Application 2002/0074541 wherein a hard layer of a magnetic device is made of nanoparticles that are separated by an insulating barrier comprised of a carbon-based coating. The free layer is formed on the insulating barrier. In this case, the nanoparticles are required to remain ferromagnetic to maintain a magnetic moment.
In U.S. Patent Application 2005/0026308, a magnetic liner is formed with super para-magnetic properties to eliminate fringing fields and hysteresis effects. Ferromagnetic films are made from ferromagnetic particles about 10 nm in size and are separated from one another by a polymer, non-magnetic metal, or an oxide.
U.S. Patent Application 2004/0023065 and a related publication “Spin-Dependent Tunneling Junctions with Superparamagnetic Sensing Layers” by D. Wang et. al, IEEE Transactions on Magnetics, Vol. 19, No. 5, p. 2812-2814 (2003) describe a super-paramagnetic (SP) free layer made of NiFeCo that is formed on a Ru bottom electrode. The Ru bottom electrode functions as a buffer layer to enable formation of uniform platelets in the overlying NiFeCo layer. However, as the device has no hysteresis, information cannot be stored therein. Thus, the scope is limited to a magnetic field sensor and does not encompass memory applications.